1. Field of the Invention
The present invention relates to a light-curable resin composition having an antistatic property, and more particularly, to a light-curable resin composition having a low shrinkage rate and an improved tilt property.
2. Description of the Related Art
A plastic material has been widely used for electrical, electronic and optical materials etc. Examples of the plastic material include vinylchloride resin (PVC), methylmethacrylate resin (PMMA), polycarbonate resin (PC), acrylonitril-butadiene-styrene resin (ABS) and polyethyleneterephthalate resin (PET). These plastic materials have various advantages such as a light weight, a low manufacturing cost, a breakage resistance, and a moldability. Thus, they are used as a substitute for glass and metal products. However, a surface of the plastic material has a poor abrasion and chemical resistance. Thus, the surface of the plastic material is generally treated or coated to improve these unfavorable properties. For example, a light-curable resin is used as a surface treatment for plastic materials. A light-curable resin composition is photosensitive, and a monomer included in the composition polymerizes to form a polymer. Its physicochemical properties, such as solubility, viscosity and adhesivity, are changed according to photopolymerization. Particularly, it changes from a liquid state to a solid state by a light irradiation in a short period of time. The light-curable resin composition has several advantages of being useful for a mass production of an article which is hardened in a short period of time, providing a good working environment because it is not a solvent-type, and providing no thermal stress to a substrate. It is also advantageous in providing a sufficient time to set a fine part in a proper position because it does not start being cured until UV is radiated on the light-curable resin composition. Further, It is desirable in adhesivity and endurance.
A transparent plastic material is used for an optical disc, a display panel for a PDP and an LCD, and safety glasses for a protective helmet. The light-curable resin composition used to protect surfaces of these products should have a high light transmittance as well as an abrasion and chemical resistance. A plastic is electrically nonconductive, it can be charged by a friction, etc. As a result, it can be polluted with dust and deteriorate in a light transmittance. Therefore, the light-curable resin composition for a surface protection should have an abrasion resistance, a solvent and chemical resistance, a high light transmittance and antistatic property.
In addition, the light-curable resin composition should have a high hardness as well as a low shrinkage rate to maintain a required abrasion resistance and to prevent a crack during a coating process. Particularly, when the light-curable resin composition is used for a protective layer the optical disc, the light transmittance should be 90% or more at a wavelength region of a laser to maintain a certain level of a recordability of the optical disc and a shrinkage rate should be 10% or less to prevent a deformation of the optical disc. The abrasion resistance should be 2H or more of the pencil hardness to prevent the optical disc from being scratched by a finger nail, etc.
A conventional light-curable acrylate coating composition having the antistatic property comprises a conductive fine particle, an acrylate oligomer, an acrylate monomer having 6 functional groups, an acrylate monomer having 3 functional groups, an acrylate monomer having a single functional group, a photoinitiator and an adhesivity enhancing agent. When an acrylate monomer having 5 or more of functional groups is contained in a composition, the monomer is very quickly polymerized, and a polymer with the high hardness is obtained because of a large number of cross-linking. On the other hand, although a resultant polymer has a shock resistance, a coated film manufactured from the composition having the polymer can be exfoliated from a disc substrate of the optical disc due to its low flexibility, and a crack can be easily made in the coated film. Thus, the conventional composition additionally includes the adhesivity enhancing agent to prevent the exfoliation of the coated film. However, the adhesivity enhancing agent may decrease the light transmittance of the coated film, and thus the recordability of the optical disc deteriorates. Further, when a monomer having 5 or more of functional groups is included in the composition, a resultant polymer may have a crack due to its high shrinkage rate, and its tilt property becomes poor. Thus, the composition is not suitable for the optical disc.
Titania is added to the conventional composition to enhance the antistatic property. However, the titania may decrease the light transmittance. Moreover, the titania is added to the composition in a powder form, thereby resulting in a non-uniform thickness of the coating film, and an adhesiveness between the disc substrate and the coating film is not good.
An antistatic composition comprising a monomer having a quaternary ammonium salt and an acrylic group may produce the coating film which shows a color change to yellow.
It is difficult to satisfy the high abrasion resistance and the low shrinkage rate simultaneously, and the conventional light-curable resin compositions have a limitation on an application field since, although they have a relatively high abrasion resistance, they do not have a high light transmittance, the low shrinkage rate and the antistatic property simultaneously. Particularly, the conventional light-curable resin compositions are not suitable for the optical disc.